Interfacial diffusion of one material within another is fundamental to many chemical separation methods as well as selected energy conversion processes. Sorption processes encompass absorption, adsorption, and desorption. Sorption may be used alone or in combination with other unit operations including but not limited to heat exchange, chemical reactions, pumping, and expansions to provide chemical products, space conditioning including cooling, power generation, or combinations thereof.
Chemical Separations
Chemical separations are industrially important processes for applications including but not limited to removing impurities from or purifying material products, removing contaminants from liquid and gas streams, and separating materials for recycle or use as byproducts to a main product stream. Conventional hardware is typically very large, measurable in tens to hundreds of cubic meters in volume, thereby necessitating the application of engineering economies of scale, in large chemical processing plants, in order to provide for cost effective chemical separations.
Interfacial Diffusion Processes
Many of the most commonly used separation processes rely upon the physical phenomena of interfacial diffusion, including sorption (e.g. adsorption, absorption, desorption), distillation, liquid--liquid extraction and combinations thereof.
The process of gas absorption involves the contact of a gas with a liquid, with one or more of the constituents of the gas being absorbed within the liquid. A phase change occurs with this process, and typically considerable heat is given off (i.e., the heat of absorption). Since the solubility of the liquid is typically inversely proportional to the temperature of the liquid, this heat can be a limiting factor in the design of gas absorption units, unless methods are incorporated for removing the heat of absorption as it is generated.
Liquid--liquid extraction is similar to gas absorption, except that both mediums are liquids. Typically, one liquid is a first solvent or first medium containing a solute, or working compound which is the material to be transferred or extracted, and the second liquid or second medium is often referred to as the solvent that receives the solute or working compound. Since no phase change occurs, there is usually very little heat generated in liquid--liquid extraction processes unless another unit operation, such as a chemical reaction, also occurs.
Adsorption is much like gas absorption into a liquid, except that the gas is now sorbed within a solid media. As with gas absorption, heat is generated as the gas is adsorbed, and this can limit the rate at which gas is adsorbed unless it is promptly removed from the adsorbing media. Adsorption further includes liquid adsorption into a solid as for an ion exchange resin. Thus, fluid adsorption as used herein includes both gas adsorption and liquid adsorption.
Desorption is generally understood as the removal of a material from a liquid stream, or from a solid media, evolving it off as a gas and is the opposite of absorption or adsorption. Multiple compounds may be observed in the effluent from a desorber. For example, when ammonia is desorbed from a liquid mixture of water and ammonia, both water and ammonia are present in the gaseous effluent. Desorption can be accomplished through the addition of heat or a change in the partial pressure of the working compound within the fluid. Note that, as used herein, desorption includes the processes that are commonly called dewatering, stripping, and dehydrating.
Distillation is the separation of miscible materials based upon differences in their boiling points. It is typically performed in multiple stages, with the vapor and liquid phases flowing countercurrently and the net effect over several or many stages can be a substantial degree of separation or purity.
All of these processes involving interfacial diffusion may also involve chemical reactions, as in reactive distillation, or may involve no chemical reactions.
In general, interfacial diffusion processes involve a phase interface (gas-liquid, liquid--liquid, gas-solid, or liquid-solid), and transport of the working compound or solute across at least one fluid boundary layer. For those not including a solid, the microscopic steps that must occur include a) transport of the working compound molecules within the bulk fluid to the boundary layer, b) transport through the boundary layer to the phase interface, c) transport of the working compound molecules across the phase interface (perhaps requiring a phase change), d) transport of the working compound molecules through the boundary layer of the solvent, and e) transport of the working compound molecules away from the fluid boundary layer.
Interfacial Diffusion Equipment
Interfacial diffusion processes for the separation of bulk chemicals have traditionally been performed by the chemical process industry through the utilization of columns, with two fluids moving in opposite directions through the column. For example, liquid--liquid extraction is performed between two immiscible fluids, typically with the lighter fluid being introduced at the bottom of the column, and the heavier fluid being introduced at the top of the column. In this example, the lighter fluid will be assumed to contain the solute, i.e., the material to be extracted, and the heavier fluid will be assumed to contain a suitable solvent. In general, a high degree of solvent loading is often desired for the effluent of the column. Herein, gravity provides a motivation for fluid flow, and as the two fluids contact each other, the solute is transferred from the feed stream to the solvent.
Disadvantages associated with the equipment example include the nonuniform fluid flow characteristics of the column and the significant time that is required to allow mass transport and then reseparation (by gravity) of the two fluids. For these reasons, the designers of separation equipment typically give strong attention to the phenomenological processes at work, especially including the application of the principles of mass transport. For example, for laminar flow conditions, mass transport is due to diffusion, with the time for a molecule to move a net distance being directly proportional to the square of the distance and inversely proportional to its mass diffusivity. Likewise, the residence time for a collective amount of mass transport and sorption has the same proportionalities, and in general, the capacity of a given piece of separation equipment is inversely proportional to residence time. Hence, the designers attempt to create geometries and flow conditions such that short residence times are characteristics of the equipment, allowing high processing rates for a given hardware volume.
Short residence times for traditional interfacial diffusion separation equipment has often been obtained through either the incorporation of actuators or packings. For example, the addition of mechanical mixing equipment is commonly used within liquid--liquid extraction units to force the creation of, and intimate contact of, thin fluid streams across which mass transport would be rapid. Or, in the case of gas absorption, where the limiting transport step is often due to the much lower diffusivity of the liquid solvent, the gas may be passed through a spray of the solvent liquid, or brought in contact with a falling film of the solvent fluid. This enables the gas, or a component of the gas, to more rapidly be sorbed and transported within the fluid film.
Engineered packings likewise are used in sorption separation equipment in order to reduce mass transport times. For example, structured packings of gauze or sheet metal are often used within distillation columns, in order to improve mass transport efficiency, and a number of designs exist for types of packings. Engineered packings have been designed and applied to distillation units, liquid--liquid extraction units, and gas absorption units. Besides reducing fluid stream thicknesses, the packing also improves uniformity of fluid flow fields, so that the processing rates are optimized throughout the hardware system. However, fluid stream thickness generally exceeds the boundary layer thickness thereby retaining all transport steps described above.
In recent years, development in advanced packings have provided significant improvements in sorption unit capacities (the rate at which material is processed) and efficiencies (product purities). For example, Humphrey and Keller ("Separation Process Technology," McGraw-Hill, 1997) refer to membrane phase contactors for absorption and stripping. In these units, hollow fiber membranes are incorporated, with one fluid stream flowing within the bore of the fibers and another on the outside of the fibers. In these cases, the membranes contain random micropores, which are filled with the liquid phase, and are typically made of materials such as polypropylene. The chief resistances to mass transport for these units occur within the membrane material, where the diffusion path is torturous and subject to fouling, and external to the unit, where the diffusion lengths are significantly longer than within the hollow fibers.
In general, the performance of interfacial diffusion units for chemical separations is chiefly limited by resistances to mass transport. "Dead space" plus nonuniform flow fields, combined with long mass transport distances, cause separation equipment to typically have residence times that are characterized by minutes or hours, thereby requiring large hardware in order to provide significant production capacity. Substantial capital investment is often required because of these inherent limitations. Often, economies can be realized only through the application of economies of scale, requiring large production capacity in order to justify the inclusion of separation equipment.
For some operations (e.g., absorption, desorption, adsorption, distillation, etc.), heat transport resistances can provide performance limits as well. As can be seen from the ongoing evolution of conventional hardware, there is generally a need for efficient fluid contact, reducing heat and mass transport resistances, through short diffusion pathways, uniform fluid flow fields, resistance to fouling, and the ability to intimately add or extract heat from the sorbing media.
State of the art chemical separations tend to be more cost effective as central systems and are less cost effective on smaller scale distributed systems.
Space Conditioning Control
Chemical processes or unit operations are further used in space conditioning or climate control hardware. Microclimate control applications include but are not limited to manportable cooling and distributed space conditioning, for example 1) vehicle space conditioning; 2) distributed cooling of buildings where the use of multiple small heat pumps may eliminate the need for ducting systems, which typically wastes 50% of the cooling produced by a central cooling system; 3) lightweight air-transportable space conditioning; 4) autonomous cooling for shipping, and 5) autonomous cooling for portable containers.
In manportable cooling situations, individuals must wear protective clothing which significantly reduces heat transfer from the body. Examples include workers exposed to hazardous materials e.g. chemicals, smoke and/or radionuclides, police wearing body armor, and individuals potentially exposed to chemical or biological agents. While protective suits provide protection against hazards, they significantly decrease an individual's effectiveness. Personnel performing labor intensive tasks in a hot environment are susceptible to heat stress, especially when wearing protective clothing. The time that can be spent performing essential tasks, before succumbing to heat injury, is limited under these conditions. Supplemental cooling will permit tasks to be performed under hazardous conditions in hot climates with enhanced efficiency and reduced heat stress. Thermodynamically, a cooling cycle is the reverse of a power cycle. Although theoretically there are many thermodynamic cycles to choose from, there are three commercially prominent thermodynamic cooling cycles in use; (1) vapor-compression which requires high mechanical work input (electricity) and which is typically physically heavy because of the need for both the cooling unit and the motor (electric); heat actuated heat pumps of two sub-types, (2) absorption from and to a liquid and (3) adsorption from and to a solid. Of course, it is well understood that a thermodynamic cycle may be operated in the reverse to convert thermal energy to shaft work.
Vapor Compression Cycle
A vapor compression cycle uses a mechanical compressor to compress a working fluid in a vapor phase. The mechanical compressor may be driven by an electric motor. As the working fluid is compressed, its temperature increases. The compressed working fluid is condensed in a heat exchanger giving up heat to the surroundings and reducing the temperature of the working fluid. The cooled working fluid is decompressed through an expander which may be an expansion valve or an orifice reducing its temperature below that of the space to be cooled. The decompressed, cooled working fluid is returned to the vapor phase by receiving heat from the space to be cooled and returns to the mechanical compressor.
Although present vapor-compression cooling systems can be integrated with protective suits and distributed spaces for cooling, the present cooling systems are too heavy to carry for extended periods. Typically, a complete system sized for 4-hour operation with a cooling capacity of 350 W weighs more than 10 kg. Vapor compression cycles require significant shaft work (or electric power) for compression of the working fluid. While gains may be made by using microcomponents, for example condensers and evaporators, the overall weight and size of a vapor compression microchannel cooling system including a motor will be larger than for a sorption cycle (absorption or adsorption) for the same thermal load.
Absorption Cycle
An absorption heat pump is similar to the vapor-compression heat pump except that the mechanical compressor in the vapor compression cycle is replaced with a chemical compressor. The chemical compressor has five components; two are chemical separation units a desorber and an absorber, expander, regenerative heat exchanger and a pump. In the desorber, a mixture of fluids (circulating fluid, e.g. lithium bromide, and refrigerant, e.g. water) is heated and the refrigerant leaves the mixture as a vapor. The refrigerant is at a high pressure in the condenser and provides cooling in the evaporator after passing the expander and reducing its pressure. The reduced pressure refrigerant is absorbed back into the circulating fluid in the absorber. The absorbed mixture is pressurized by a pump and returns to the desorber, preferably through a regenerative heat exchanger. Because the mixture is a liquid, pump work is typically about 1/100 of the amount of work (electricity) required to compress a vapor. Thus, the absorption cycle trades lower electricity requirement when compared to the vapor compression cycle. However, the absorption cycle does require a source of thermal energy. There are many variations of absorption cycles including but not limited to single effect, double effect, Generator/Asorber/Heat Exchanger (GAX), Diffusion Absorption and combinations thereof.
A conventional absorption cycle system relies on gravity to form falling films, which provide liquid to gas contact in the absorber and desorber. This approach has two decisive disadvantages for many portable space conditioning applications. First, the heat pump must be oriented so that the solution will fall over heat exchanger tubes and form a thin film. Deviations from the proper orientation will prevent the heat pump from working. Second, falling films have a film thickness on the order of 1 mm preventing effective mass transfer via diffusion and resulting in a physically large absorber and desorber. For distributed space cooling, weight is not as significant a factor as for manportable cooling, but for vehicle cooling including transportation containers and aircraft, weight reduction is an important consideration.
Although the absorption and vapor compression cycles differ in the way compression is provided, both systems take the same approach to heat absorption and rejection. In both cycles, superheated refrigerant enters the condensing heat exchanger, where it undergoes constant-pressure heat rejection. The resulting condensate or mixture of condensate and vapor is then adiabatically expanded through either a throttling valve or a capillary. The mixture is then routed to an evaporating heat exchanger for constant-pressure heat absorption. Compression is accomplished in the absorption heat pump system with a through single effect thermochemical compressor or desiccator consisting of an absorber, a solution pump, a regenerative heat exchanger, and a desorber (gas generator).
Absorption cycles may be grouped based on the fluid combination and on cycle arrangements. The most widely used fluid combinations are lithium bromide (LiBr) and water, where water is the refrigerant; and water and ammonia (NH.sub.3), where ammonia is the refrigerant. Cycle arrangements include the single-effect cycle described above to progressively more efficient but complicated multiple effects, for example a double-effect cycle.
The single-effect LiBr/H.sub.2 O cycle requires a low pressure solution pump (approx. 41 kPa (6 psi) pressure rise), but the cycle is less efficient than the double-effect cycle. While more efficient, the double-effect LiBr/H.sub.2 O cycle requires a higher pressure pump (approx. 410 kPa (60 psi) pressure rise) and is more complicated than the single-effect cycle. The pressure rise required for a H.sub.2 O/NH.sub.3 solution pump (2400 kPa, 350 psi) is too high for currently available small pumps and results in a heavy and inefficient system. Thus, both the single-effect and the double-effect LiBr/H.sub.2 O absorption cycles are preferred candidates for cooling applications where weight and size are key issues. The H.sub.2 O/NH.sub.3 system is needed in cases requiring both heating and cooling or requiring cooling below 0.degree. C. (32.degree. F.).
A microchannel condenser and evaporator has been demonstrated by Cuta, J. M., C. E. McDonald, and A. Shekarriz. 1996. "Forced Convection Heat Transfer in Parallel Channel Array Microchannel Heat Exchangers." Advances in Energy Efficiency, Heat/Mass Transfer Enhancement PID-Vol. 2HTD-Vol. 338, American Society of Mechanical Engineering, New York, also in U.S. Pat. No. 5,611,214 both herein incorporated by reference. Briefly, the microchannel condenser consists of an array of microchannels with channel widths between 100 and 300 microns and channel depths up to 1 mm. Heat transfer rates in excess of 30 W/cm.sup.2 were attained with a small temperature difference and low pressure drop. The microchannel evaporator also consists of an array of microchannels with channel widths between 100 and 300 microns and channel depths up to 1 mm. Results show that convective heat transfer coefficients of 1.0 to 2.0 W/cm.sup.2 -K are readily attainable, and heat transfer rates up to 100 W/cm.sup.2 can be obtained with a small temperature difference. These heat transfer coefficients and rates exceed those of conventional evaporators by a factor of 4 to 6. Pressure drop is typically less than 6 kPa (1 psi).
Absorption systems require a heat source for the desorber. Drost, M. K. C. J. Call, J. M. Cuta, and R. S. Wegeng. 1996. "Microchannel Integrated Evaporator/Combustor Thermal Processes." Presented at 2nd U.S. Japan Seminar in Molecular and Microscale Transport Phenomena, August 8-10, Santa Barbara, Calif., and U.S. application Ser. No. 08/883,643, both incorporated by reference herein. The microchannel combustor produces thermal energy at a rate of at least 30 W/cm.sup.2, with a thermal efficiency between 82 and 85%.
Absorption system efficiency is increased with a regenerative heat exchanger, wherein no phase change occurs for the working fluid or the heat transfer fluid. Non-phase change microchannel heat transfer is well known, for example Ravigururajan, T. S., J. Cuta, C. McDonald and M. K. Drost. 1995. "Single Phase Flow Thermal Performance of a Parallel Micro-Channel Heat Exchanger," Presented at American Society of Mechanical Engineers 1995 National Heat Transfer Conference. Microchannels with channel widths between 100 and 300 microns and channel depths up to 1 mm provide single-phase microchannel heat transfer convective heat transfer coefficients of 1.0 to 1.2 W/cm.sup.2 -K. These heat transfer coefficients exceed conventional regenerative heat exchanger performance by a factor of 3 to 6.
Adsorption Cycle
Adsorption cycle systems rely on the adsorption of a refrigerant into a solid to provide heat pumping. A typical system would contain two pressure vessels filled with the adsorbent. The adsorbent in one vessel has adsorbed a refrigerant. When that vessel is heated the refrigerant is desorbed from the solid adsorbent at a high pressure, the refrigerant is cooled to ambient temperature and then passes through an expander (orifice or expansion valve) where the pressure (and consequently temperature) of the refrigerant is reduced. Thermal energy is then transferred from the cooled space to the refrigerant and the refrigerant is then adsorbed in the adsorbent in the second pressure vessel. The second vessel is cooled to remove the heat of adsorption and to maintain a low pressure. This continues until the entire inventory of refrigerant has left the first tank and is adsorbed in the second tank. At this point the process is reversed and the second tank is heated, driving off the refrigerant which provides cooling and is returned to the first tank. There are many variations on the adsorption cycle and a wide range of materials has been investigated for adsorbents and refrigerants. However, all of these systems rely on adsorption in one form or another. While the adsorption cycle has been extensively investigated in both the U.S. and in foreign countries the concept has had practically no impact and there are few commercially available systems. Problems with adsorption systems include difficulty in adding or removing thermal energy from the adsorbent, the large inventory of adsorbent required and degradation of the adsorbent by repeated cycling.
Need
There exists a need for a fundamental method and apparatus that would overcome the inherent heat and/or mass transport limitations of state of the art chemical separations and cooling systems that would permit more compact distributed chemical separations and permit man-portable cooling systems.